1. Field of the Invention
This invention relates to a heat-insulating piston structure for a heat-insulating engine.
2. Description of the Prior Art
A conventional engine member of a heat-insulating piston in which a ceramic material is utilized as a heat-insulating material and a heat resisting material is disclosed in, for example, Japanese Utility Model Laid-Open No. 113557/1984 and Japanese Patent Laid-Open No. 93161/1985.
First, the construction of the piston disclosed in Japanese Utility Model Laid-Open No. 113557/1984 will be roughly described with reference to FIG. 3. FIG. 3 shows a piston 30. In this piston 30, a ceramic crown portion 31 and a metallic skirt portion 32 are combined together by a bolt 36 so that a closed space 33 is formed between the lower surface of the ceramic crown portion 31, which has a combustion chamber 39 in the upper surface thereof and a groove 37 for a piston ring in the outer circumferential surface thereof, and the upper surface of the metallic skirt portion 32, which has grooves 38 for further piston rings in the outer circumferential surface thereof, and a seal member 35 is provided so that it can be engaged with an end portion of a piston pin inserting bore 34 opened in the outer circumferential surface of the metallic skirt portion 32. In this piston structure, the crown portion consisting of a ceramic material has an extremely large thickness, and, therefore, its required thermal capacity becomes very large. Since the combustion chamber 39 is formed in the crown portion 31, it is necessary that the crown portion 31 be formed to a large thickness to maintain the construction 1 characteristics and a suitable strength thereof.
The construction of the heat-insulating piston disclosed in Japanese Patent Laid-Open No. 93161/1985 will now be roughly described with reference to FIG. 4. FIG. 4 shows a heat-insulating piston designated generally by a reference numeral 40. In this heat-insulating piston 40, a crown fitting bore 43 is provided in an upper end wall 52 of a piston body 50 which includes a piston skirt portion 42 having piston ring fitting grooves 49 and a piston pin fitting bore 51, and a projection 44 formed on a crown 41 is inserted in the bore 43, the portion of the piston body 50 which is around the bore 43 being thermally pressed to combine the piston body 50 and crown 41 with each other. The piston body 50 is formed out of aluminum or malleable cast iron, and the crown 41 out of a ceramic material, such as silicon nitride. The projection 44 of the crown 41 is provided with a combustion chamber 47 formed in the interior thereof, and a smaller projection 45 is formed on the outer circumferential portion of the crown 41. A heat-insulating material 46 consisting of ceramic fiber or a stainless steel mesh arranged in a hollow 48 formed between the projections 44, 45 is fixed in a sandwiched state between the relative portions of the crown 41 and the upper end wall 52 of the piston body 50. The heat-insulating characteristics of this heat insulating material 46 displayed with respect to the combustion chamber 47 are not satisfactory. Moreover, the thickness of the crown 41 consisting of a ceramic material is very large similarly to that of the crown portion 31 of the previously-described piston 30, and the crown 41 is formed in such a manner that the crown 41 is exposed directly to the heat in the combustion chamber 47. This causes the required thermal capacity of the piston to increase.
It is very difficult to furnish a heat-insulating piston member, which utilizes the above-mentioned ceramic material as a heat-insulating material or a heat resisting material, with satisfactory heat-insulating characteristics. Since the ceramic material is exposed to the high-temperature heat in the combustion chamber, it receives a thermal shock. Therefore, it is necessary that the member consisting of a ceramic material be formed to a preferable strength. If the thickness of the ceramic material constituting the wall of the crown is increased for the heat-insulating purpose, the thermal capacity of the wall becomes large. Accordingly, in a suction stroke, the suction air receives a large quantity of heat from the combustion chamber to cause the temperature of the suction air to increase, so that this heat adversely affects the air suction operation. As a result, the suction efficiency decreases, and the air suction operation stops. Moreover, it is necessary that the heat-insulating characteristics of the member of a ceramic material with respect to the heat occurring in an expansion stroke be improved.